We determine unsteady time-periodic flow perturbations that are optimal for enhancing the time-averaged rate of heat transfer between hot and cold walls (i.e. the Nusselt number Nu), under the constraint of fixed flow power (Pe$^2$, where Pe is the Péclet number). The unsteady flows are perturbations of previously computed optimal steady flows and are given by eigenmodes of the Hessian matrix of Nu, the matrix of second derivatives with respect to amplitudes of flow mode coefficients. Positive eigenvalues of the Hessian correspond to increases in Nu by unsteady flows, and occur at $Pe\geqslant 10^{3.5}$ and within a band of flow periods $\tau \sim Pe^{-1}$. For $\tau {\textit{Pe}}\leqslant 10^{0.5}$, the optimal flows are chains of vortices that move along the walls or along eddies enclosed by flow branches near the walls. At larger $\tau {\textit{Pe}}$, the vorticity distributions are often more complex and extend farther from the walls. The heat flux is enhanced at locations on the walls near the unsteady vorticity. We construct an iterative time-spectral solver for the unsteady temperature field, and find increases in Nu of up to 7 % at moderate-to-large perturbation amplitudes.

Shock-tube experiments are conducted to investigate the Atwood-number dependence of hydrodynamic instability induced by a strong shock with a Mach number exceeding 3.0. The compressible linear theory performs reliably under varying compressibility conditions. In contrast, the impulsive model significantly loses predictive accuracy at high shock intensities and Atwood numbers ($A_t$), particularly when specific heat ratio differences across the interface are pronounced. To address this limitation, we propose a modified impulsive model that offers favourable predictions over a wide range of compressibility conditions while retaining practical simplicity. In the nonlinear regime, increasing $A_t$ enhances both the shock-proximity and secondary-compression effects, which suppress bubble growth at early and late stages, respectively. Meanwhile, spike growth is promoted by the spike-acceleration and shock-proximity mechanisms. Several models reproduce spike growth across a wide range of $A_t$, whether physical or incidental. In contrast, no models reliably describe bubble evolution under all $A_t$ conditions, primarily due to neglecting compressibility effects that persist into the nonlinear regime. Building on these insights, we develop an empirical model that effectively captures bubble evolution over a wide $A_t$ range. Modal evolution is further shown to be strongly affected by compressibility-induced variations in interface morphology. The effect is particularly pronounced at moderate to high $A_t$, where it suppresses the fundamental mode growth while promoting higher-order harmonic generation.

Jet vortex generators (JVGs) are a promising technique for controlling laminar separation in low-Reynolds-number aerofoils, such as those used in micro air vehicles (MAVs). While previous studies have demonstrated their aerodynamic benefits, the three-dimensional structure of the vortices they generate and their interaction with the boundary layer remain poorly characterised experimentally. In this study, volumetric velocity measurements are performed using the double-pulse Shake-the-Box (STB) technique on an SD7003 aerofoil equipped with skewed and pitched JVGs. Experiments are conducted at Reynolds numbers of 30 000 and 80 000, for angles of attack of 8$^{\circ}$, 10$^{\circ}$ and 14$^{\circ}$. The results provide the first experimental visualisation of the full three-dimensional vortex topology induced by JVGs, revealing asymmetric streamwise vortices that penetrate the separated shear layer and re-energise the near-wall region. In pre-stall conditions, the JVGs reshape the laminar separation bubble into a thinner and more stable structure, reducing its sensitivity to angle of attack. In stall conditions, they induce partial or full flow reattachment, delaying large-scale separation. The evolution of characteristic bubble parameters and the chordwise distribution of the shape factor $H = \delta ^{\ast }/\theta$, where $\delta ^{\ast }$ is the displacement thickness and $\theta$ is the momentum thickness, show a consistent trend of enhanced boundary-layer recovery. These findings offer new insight into the physical mechanisms underlying active separation control at low Reynolds numbers and establish a framework for evaluating vortex-based control strategies using volumetric diagnostics.

Microswimmers and active colloids often move in confined systems, including those involving interfaces. Such interfaces, especially at the microscale, may deform in response to the stresses of the flow created by the active particle. We develop a theoretical framework to analyse the effect of a nearby membrane on the motion of an active particle whose flow fields are generated by force-free singularities. We demonstrate our results on a particle represented by a combination of a force dipole and a mass dipole, while the membrane resists deformation due to tension and bending rigidities. We find that the deformation either enhances or suppresses the motion of the active particle, depending on its orientation and the relative strengths between the fundamental singularities that describe its flow. Furthermore, the deformation can generate motion in new directions.

The interaction between a coherent vortex ring and an inertial particle is studied through a combination of experimental and numerical methods. The vortex ring is chosen as a model flow ubiquitous in various geophysical and industrial flows. A detailed description of the vortex properties together with the evolution of the particle kinematics during the interaction is addressed thanks to time-resolved particle image velocimetry and three-dimensional shadowgraphy visualisations. Complementary, direct numerical simulations are realised with a one-way coupling model for the particle, allowing for the identification of the elementary forces responsible for the interaction behaviours. The experimental and numerical results unequivocally demonstrate the existence of three distinct interaction regimes in the parameter range of the present study: simple deviation, strong deviation and capture. These regimes are delineated as functions of key controlled dimensionless parameters, namely, the Stokes number and the initial radial position of the particle relative to the vortex ring axis of propagation.

High Reynolds number effects of wall-bounded flows, involving interscale energy transfers between small and large scales of turbulence within and between the inner and outer regions, challenge the classical description of the structure of these flows and the ensuing turbulence models. The two-scale Reynolds stress model recently proposed by Chedevergne et al. (2024, J. Fluid Mech. vol. 1000), was able to reproduce the small- and large-scale contributions in turbulent channel flows that follow the scale separation performed by Lee & Moser (2019, J. Fluid Mech. vol. 860, pp. 886–938), by partitioning energy spectra at a given wavelength. However, the interscale interactions within the inner region were modelled in an ad hoc manner, but without physical relevance, making the two-scale Reynolds stress model less and less accurate for boundary layer applications as the Reynolds number was increased. In this study, by re-analysing direct numerical simulations data from Lee & Moser (2019), with the objective of modelling these scale interactions, crucial observations on energy transfers between large and small scales could be made. In particular, the analysis reveals the important role played by the spanwise component of the Reynolds stress in the logarithmic region. From the analysis undertaken, a revisited version of the two-scale model was thus proposed, focusing efforts on interscale transfer modelling. The resulting model is then successfully tested on high Reynolds number boundary layer configurations without pressure gradient, up to $\textit{Re}_{\tau }=20\,000$. The excellent agreement reflects the good prediction capabilities of the proposed model, and above all, the relevance of the modelling of the energy transfers within and between the inner and outer regions of wall-bounded flows.

The flow in a rapidly rotating cylinder forced by the harmonic oscillations of a small sphere along the rotation axis is explored numerically. For oscillation frequencies less than twice the cylinder rotation frequency, the forced response flows feature conical shear layers emitted from the critical latitudes of the sphere. These latitudes are where the characteristics of the hyperbolic system, arrived at by ignoring nonlinear, viscous and forcing terms in the governing equations, are tangential to the sphere. These conical shear layers vary continuously with the forcing frequency so long as it remains inertial. At certain values of the forcing frequency, linear inviscid inertial modes of the cylinder are resonated. Of all possible inertial modes, only those whose symmetries are compatible with the symmetry of the forced system are resonated. This all occurs even in the linear limit of vanishingly small forcing amplitude. As the forcing amplitude is increased, nonlinearity leads to non-harmonic oscillations and a non-zero mean flow which features a Taylor columnar structure extending from the sphere to the two endwalls in an axially invariant fashion.

Pulsatile fluid flows through straight pipes undergo a sudden transition to turbulence that is extremely difficult to predict. The difficulty stems here from the linear Floquet stability of the laminar flow up to large Reynolds numbers, well above experimental observations of turbulent flow. This makes the instability problem fully nonlinear and thus dependent on the shape and amplitude of the flow perturbation, in addition to the Reynolds and Womersley numbers and the pulsation amplitude. This problem can be tackled by optimising over the space of all admissible perturbations to the laminar flow. In this paper, we present an adjoint optimisation code, based on a GPU implementation of the pseudo-spectral Navier–Stokes solver nspipe, which incorporates an automatic, optimal checkpointing strategy. We leverage this code to show that the flow is susceptible to two distinct instability routes: one in the deceleration phase, where the flow is prone to oblique instabilities, and another during the acceleration phase with similar mechanisms as in steady pipe flow. Instability is energetically more likely in the deceleration phase. Specifically, localised oblique perturbations can optimally exploit nonlinear effects to gain over nine orders of magnitude in energy at a peak Reynolds number of ${\textit{Re}}_{\textit{max}}\approx 4000$. These oblique perturbations saturate into regular flow patterns that decay in the acceleration phase or break down to turbulence depending on the flow parameters. In the acceleration phase, optimal perturbations are substantially less amplified, but generally trigger turbulence if their amplitude is sufficiently large.

In this paper, we consider the flow of a nematic liquid crystal in the domain exterior to a small spherical particle. We work within the framework of the $\unicode{x1D64C}$-tensor model, taking into account the orientational elasticity of the medium. Under a suitable regime of physical parameters, the governing equations can be reduced to a system of linear partial differential equations. Our focus is on precise far-field asymptotics of the flow velocity with an emphasis on its anisotropic behaviour. We are able to analytically characterize the flow pattern and compare it with that of the classical isotropic Stokes flow. The expression for velocity away from the particle can be computed numerically or symbolically.

Uniform momentum zones (UMZs) are widely used to describe and model the coherent structure of wall-bounded turbulent flows, but their detection has traditionally relied on relatively narrow fields of view which preclude fully resolving features at the scale of large-scale motions (LSMs). We refine and extend recent proposals to detect UMZs with moving-window fields of view by including physically motivated coherency criteria. Using synthetic data, we show how this updated moving-window approach can eliminate noise contamination that is likely responsible for the previously reported, high fractal dimension of UMZ interfaces. By applying the approach to channel flow direct numerical simulation (DNS), we identify a significant number of previously undetected, large-scale UMZ interfaces, including a small fraction of highly linear interfaces with well-defined streamwise inclination angles. We show that the inclination angles vary inversely with the size of the UMZ interfaces and that this relationship can be modelled by the opposing effects of shear-induced inclination and vortex-induced lift-up on hairpin packets. These geometric properties of large-scale UMZ interfaces play an important role in the development of improved stochastic models of wall-bounded turbulence.